I have one 500 GB HDD for my photographs (digital and analog) libraries and editing documents, one 500 GB HDD with my personal video footage in an editing friendly format.
Both 500 GB HDDs get backed up to one 1 TB HDD via CarbonCopyCloner.
And that 1 TB HDD gets backed up to another 1 TB HDD via CarbonCopyCloner.
Therefore I have three copies of my important data.

My old macbook air and external hard drive (which contained the majority of my photos) were both backed up to a 1TB external hard drive on time machine.

But only the macbook was stolen. Does anyone know if I must have my external hard drive connected when I restore my mac from time machine? Or does it differentiate between the two? I feel like it would be unnecessary (and slow) to back up both. Plus the chance for failure is higher, since there's no reason for me to completely wipe my external hard drive just to restore all the data that is already on it...

I tried this set up with an external HDD connected to my 128GB rMBP. Unfortunately, Time Machine attempts to backup my ENTIRE external drive every time it's connected to my Mac.

This seems to happen because the most recent backup happened when the external HDD was not attached, so Time Machine apparently thinks the external is "new" every time. Like, Time Machine doesn't remember it backed up the external a few days ago! How can I fix this???

What to Do if My External Hard Drive is in the Time Machine Excluded List?

I saw earlier that someone said that you can back up an external hard drive to Time Machine as long as it's not in the excluded list. Unfortunately, my external hard drive is in the excluded list and I have tons of irreplaceable files on it already so therefore can't wipe it and reformat it (I think). I tried to remove it from the exclusion list with no luck: the minus link is grayed out for it. Any suggestions?

I saw earlier that someone said that you can back up an external hard drive to Time Machine as long as it's not in the excluded list. Unfortunately, my external hard drive is in the excluded list and I have tons of irreplaceable files on it already so therefore can't wipe it and reformat it (I think). I tried to remove it from the exclusion list with no luck: the minus link is grayed out for it. Any suggestions?

Open Disk Utility and check what format the external drive is in. It will need to be Mac OS Extended for TM to back it up. Sounds like maybe your external is in some other format.